I am posting here beacause I would like some well thought out solutions to this problem.

The problem is with my new motor build (see mods below) stumbling and backfiring thru the intake at part throttle, thru all RPM ranges, and also when reving the motor without a load. The motor idles great and has power after the stumble and runs great even thru the backfires (just sounds like it's gonna blow something apart).

When the motor had 0 miles on it I didn't notice this problem and it has gotten to the point now where I can't drive it at only 300 miles. I also had a problem with my fuel pressure bleeding off every time I shut the car off, so I upgraded the fuel pump and changed out the regulator and it still did not fix the bleeding fuel pressure issue. I believe I may have an bad injector, and am tring to pin the backfires on it, but can't seem to think why this would happen. (Possibly fuel dripping down on a hot exhaust valve during the overlap?)

These are the components that have been eliminated as source of the problem. (Note all components were changed out with new or good components)

Cam alignment - Tripple checked cam during rebuild.
Valve ajustment - Reajusted all valves using 1/4 turn for proper Pre-load for crane lifters
Optispark - Brand new unit 1500 miles (Tried the old unit which still worked)
Plug Wires - Replaced MSD wires with Brand new taylor set (Tripple checked plugs wires to opti)
Coil - Older MSD coil (Used new cheap autozone coil)
Ignition Module - Changed out stock unit for autozone unit

I am running out of compnents to check. (Any help would be awsome)

Thanks
Jeff

 

Which Crane lifters do you have? How much pre-load do they require?

Did you use the baseline method to set valves, or set them in batches?

Pre-load may not be the root cause, but it could certainly add to the issue.

I see you have a mail order tune. Was that tune configured before, or after
you installed the camshaft?

Do you have a timing gun with an advance dial? Check the base timing and
timing at part throttle.

How about fuel pressure at idle and part throttle (under load)? Since you've
already identified a fuel issue, you might want to pay close attention to the
rail pressure.

Also stick on a vacuum gauge. You might have a vacuum hose that is loose
fitting on a port, or cracked causing a huge leak. Maybe you've left a hose
off, or improper connection. Pretty common after a rebuild.

Hope that opens some more doors.

 

As frustrating as the problem may be, this is still "LT1/LT4 Engine Tech" material. Most of the people with good LT1 knowledge respond to posts on both forums. "Advanced Tech" is not the right place.

Moving......

 

I know this probably wouldn't cause your problem, but I have heard from a post in general f-body forum that the taylor wires are known to be very faulty. Check if they are arching. Probably not your problem again, but may be adding to it.

 

I am running the crane stock replacemnet lifters that require .02" - .06" preload, and I set them in batches using #1 & #6 @ TDC. When I reajusted them the motor did seem to idle smoother.

The tune was configured for the cam after the install and it has been the only tune run on the new motor. I have two others to try that have timing pulled, but for the same setup. I also thought I might try to get a tune for the stock injectors and run them.

Thanks for the advice...

 

1/4 turn is probably more like 0.010", but that's not the worst of your worries.
If you had to adjust them again, maybe an 1/8th turn after zero lash would
get you in the window.

This part I don't agree with. I have always been taught to use the baseline
method - ESPECIALLY with aftermarket cams with big durations.

Using a batch setting may not ensure that the lifter is on the baseline of the
lobe.

Maybe this is the cause for your backfiring?

You should re-adjust the valve individually for best results.

 

At this point I have no problem re-ajusting the valves as many times as it takes and agree with you that it may not be as accurate to ajust in batches. I just hope that it can be something as simple as this.

Also I ran through the math with 7/16-20 studs, which is about .050"/ turn, and makes 1/4 turn about .013". With 1.6 Rockers the tip of the rocker (pushrod end) will move about .020" and with the pushrod at about a 30 deg angle to the Rocker this will give approx .017" of lifter preload. If I only go 1/8 of a turn that will be about .009" and under the .020" minimum. So what might happen If i under adjust them? Do I run the risk of loosing a pushrod into the motor, or worse breaking something?

 

If you have a dial indicator, set up the pin to read the primary side of the rocker arm
as you adjust pre-load.

This will give you a fairly accurate reading of how much the lifter seat is
pre-loading.

Once you set one valve with the dial, you can observe the wrench angle
and apply the same amount to the remaining valves.

I actually thought you were using a different Crane lifter which requires less
than 0.010 inch preload.

I wont speak for Crane, but I`m able to get better idle vacuum, and power
by setting my hydraulic lifters to very modest amounts of pre-load.

Under-adjusting the lifter will be tough. With a roller valve train, you wont
need to worry about much material wearing away. 0.002-0.004 is about one
sheet of paper, maybe two. That`s the visual you need.

Pre-load is used to take up slack in the valve train when parts wear down.

More pre-load allows the valve lift to vary by greater amounts. This acts
negatively on max valve lift.

I wont speak for your lifters, but you might want to ask Crane Tech., or the
boys in Advnaced Tech. about min. pre-load for your setup.

 

i had a similar problem it turns out that the dowel pin on the front of my cam was less then 1/16 of an inch too long and in ground into the back of the opti and eventually seized the little bearing that is back there, and it ran fine until exactly 350 miles after cam instal/ complete rebuild (i spun like 6 bearings had block machined and rods reconditioned and threw a GTP cam in while motor was out) i havent fixed the problem yet, cam is going to machine shop tomorrow morning and i'll let you know if that fixes my low idle stumble, i assume it will though lol

 

So I stuck the vacuume gauge on and was recording about 14-15 in Hg at idle. When I rev the motor up with no load the vacuume gauge reads around 20-21 in Hg. Does this behaivior sound right? And what should the vacuume be for the Hot Cam?

I also re-ajusted the values bringing each cylinder to TDC and ajusting both intake and exhaust valves. And changed out the injectors back to the stock ones and ran a program that was modified for it. I also ran a previous program that ran good before I tore it down. Still no luck... Car runs supper strong but the backfires are hard to deal with.

So I still need to check out the fuel rail pressure (waiting for parts store to get a gauge in). I was also wondering if my manifold could be positioned too high due to the thinner head gaskets (.026 from .039), and the machining done to the heads and the block (.015" off the heads and .010" off the block). When I put the bolts in they were tight against the manifold but I got them in so I thought it was fine. Could it be that the manifold didn't get a good seal? Just a thought.

 

Yes.

Not sure. Depends on many things like elevation, rocker ratio, pre-load setting.

I'm thinking 14" is a touch high, but you must be very close to sea level?
In any case, more vacuum is a good thing in perspective.

That's a good method as long as you can ensure TDC for each piston.
Did you use a degree wheel, or just watch the valves?


Maybe. Did you glue down the gaskets between the intake ports? If you've
had porting, it's very possible that there is a leak between runners.

As for machining the heads, and thinner head gaskets, there's also a chance
the lower portion of the port is leaking.

Are you getting oil on the plugs (from the lifter valley)?

If all of the vaccum hoses have been double and triple checked, all gaskets
are tight...the cam is phased, the valves were set on the baseline of the
cam...then I'm lost for words?

Get a timing light on #1 and see what the initial setting is.

I don't think you can do part throttle timing on EFI motors because the computer
will account for engine load (unlike mechanical advance on HEI type distributors).

 

I think that your idle vaccum reading is too low. You might check with gm for the correct idle vaccum reading with that "Hot Cam'. Try www.gm g oodwrench.com they are very helpfull. the correct vaccum reading for a stock cam engine is 15-22 in. steady reading. check all vaccum hoses,Idle air control and check torque settings of manifold. Back firing can also a leaking valve, one that is not seating properly. also a lean condition can exibit these symptoms. Using a OBD scantool watch the performance of the O2 sensors. Last but not least you can perform a "leak down" test to determine the combustion chamber condition ie valve seating. How about the fuel pressure and the condition of the fuel filter. Good luck I hope some of these suggestions help you to resolve your problem. 93Zted.

 

I decided to pull the intake manifold off, and send off the intake manifold to be machined to acount for the height changes I made to the heads and gaskets. When I put it back on I glued the gaskets down and torqued the manifold down using 5 ft/lb increments. I was wrong to think that this would help... Still backfires and vacuume is the same at 14 in at idle.

I also checked my fuel pressure at WOT under a load and it is right around 42 psi. I also ran a program that was good for using stock injectors and the stock injectors do the same thing.

So what am I missing? This is getting very agravating. Could I have a bad valve spring? (They are new Hot cam springs). Or could this be a problem running the hot cam? At this point would it make sense to advance the cam 3 deg to get the exhuast valve closed earlier?

Again a quick re-cap on what I've checked/replaced/re-ajusted

Cam alignment - Tripple checked cam during rebuild.
Valve ajustment - Reajusted all valves twice using 1/4 turn for proper Pre-load for crane lifters
Optispark - Brand new unit 1500 miles (Tried the old unit which still worked)
Plug Wires - Replaced MSD wires with Brand new taylor set (Tripple checked plugs wires to opti)
Coil - Older MSD coil (Used new cheap autozone coil)
Ignition Module - Changed out stock unit for autozone unit
Fuel Regulator - Changed to a new auto-zone regulator
Fuel Pump - Changed stock unit to a high output pump
Fuel pressure - checked pressure at WOT = 42 psi
Injectors - Ran old stock injectors
Manifold - Machined for proper fit

Again the car runs strong and pulls hard until it backfires.

Thanks